The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This invention relates to compositions and methods for restoring zonal isolation in downhole environments, and more particularly to repairing faults in cement sheaths in order to restore zonal isolation. The invention finds particular application in wells of the type used in the oil and gas industry.
Squeeze or remedial cementing is a common operation in the petroleum industry and is used to address incomplete zonal isolation in oil and gas wells. Incomplete zonal isolation can arise from cracks, voids and channels resulting from improper primary cement job design or execution, and/or stresses applied to the set cement in the annulus arising from pressure and temperature changes during the well life.
Traditionally, formulations comprising Portland cements such as API/ISO Class A or H are employed in remedial-cementing operations. However, situations frequently arise in which the cement particles are too large to enter the cracks, voids and channels. In these cases the remedial treatments are unsuccessful. These problems are more thoroughly discussed in the following publication: Daccord G et al.: “Remedial Cementing,” in Nelson EB and Guillot D (eds.): Well Cementing, 2nd Edition, Houston: Schlumberger (2006) 503-547.
Too address these problems, microcement systems have been developed to repair cracks and other voids in cement sheaths. Special formulations with microcements having optimized particle size distributions are able to penetrate cracks with widths as low as approximately 100 micrometers. While this advance has improved the general success rate of remedial cementing operations, situations continue to arise during which even microcement particles are too large, and slurries containing them are ineffective. As a result, a need remains for compositions capable of entering and sealing cracks, voids or channels smaller than 100 micrometers.
Epoxy resins are thermosetting polymers that cure (polymerize and crosslink) when mixed with a catalyzing agent or “hardener. Most common epoxy resins are produced from a reaction between epichlorohydrin and bisphenol-A. The applications for epoxy-based materials are extensive and include coatings, adhesives and composite materials. The chemistry of epoxies allows the production of resins with a very broad range of properties. In general, epoxies are known for their excellent adhesion, good mechanical properties, and chemical and heat resistance.
In oilfield and downhole environments, epoxy resins have been used for many different applications. Applications include coatings for proppants and other particulate materials (for example in US20070293402A1, US20070160755A1, U.S. Pat. No. 7,135,231B1, US20070023187A1, US20080110623A1, US20060258546A1, WO2000005302A1 and WO2002026656A1), sand control (WO2007091007A1, U.S. Pat. No. 5,178,218A, U.S. Pat. No. 5,101,900A), formation consolidation (U.S. Pat. No. 5,839,510A, U.S. Pat. No. 7,114,570B2, WO2002046574A1, WO2004022914A1, EP266043B1) and drilling (US20070021309A1, U.S. Pat. No. 6,739,392B2).
Other documents describe the use of epoxy resins in the context of zonal isolation (U.S. Pat. No. 5,547,027A, US20080023205A1 and WO2001090531A1, U.S. Pat. No. 5,377,757A, US20060234871A1, and U.S. Pat. No. 6,802,375B2). Although such materials can solve some of the problems encountered with traditional cement sealants, they present some drawbacks. In U.S. Pat. No. 5,547,027A, solvent-based resins are used, and a preflush may be required to obtain an adequate bond. This can be costly, time consuming and difficult from an operational point of view. In other patents, aqueous or water-compatible epoxy resins are employed to repair or plug openings. In these cases, in which the well temperature affects the curing of the resin, controlling the rheological properties and the setting time can be difficult.
The present invention overcomes the aforementioned problems associated with conventional Portland cement and microcement systems, and also addresses the difficulties that have arisen with other uses of epoxy resins for achieving zonal isolation.